U.S. Pat. Nos. 1,892,217 and 2,028,407, to R. J. L. Moineau, disclose a gear mechanism for use as a progressive cavity pump or motor. In a typical application of progressive cavity technology, the drilling of subterranean wells, a progressive cavity motor is used as a downhole motor to convert the energy of a flowing drilling fluid to mechanical power to rotate a drill bit.
In a progressive cavity pump or motor, both the stator and the rotor are formed with helical lobes. An interference fit between the external profile of the rotor and the internal profile of the stator provides a seal isolating the cavities of the pump or motor from adjoining cavities. The seal resists the fluid pressure which results from the mechanical pumping action, or from the conversion of fluid motion to mechanical energy in a motor.
Because of the requirement for an interference fit between the rotor and stator, one or both of these components must be covered with a resilient, or dimensionally forgiving, material, usually an elastomer, which also allows the pump or motor to pass or transfer abrasive particles and other objects carried along with the fluid. Historically, the resilient material has been provided on the interior of the stator. The rotor is coated with hard chromium to increase the wear resistance of its contacting surface.
In order to minimize friction where the rotor contacts the elastomer on the inside of the stator, the rotor must have a very highly polished surface. Currently a conventional milling process is used to generate the required outside profile along the length of the rotor. A polishing operation is then carried out to change the relatively rough surface resulting from the milling operation to an acceptable finish for chrome plating. Stainless steel alloys such as 17-4PH are often used to manufacture the rotors, because of their corrosion resistance, and their relatively easy machining.
The rotors of progressive cavity pumps typically have a bearing journal at one end, and therefore cannot be shaped by extrusion. The helical lobes of the rotor typically extend from a first end toward the second end, but stop short of the second end to allow for a bearing journal and attachment features. The rotor may be solid, or may have a hole bored partially or totally through its length. Rotors with more than one lobe have multiple concave areas that stop at some point along the length of the rotor, thus limiting the ways in which they can be manufactured.
Many other bar-like products also have external profiles that do not extend along their entire length. They have concave areas that also limit the method of manufacture. Thus this invention, while described in the context of the manufacture of a rotor for a progressive cavity fluid device, has potential applications in the manufacture of various other products.